1. Field of the Invention
The invention relates to an image recording apparatus for recording an image by using a plurality of recording elements.
2. Related Background Art
Hitherto, in such a kind of apparatus, a multi recording head in which a plurality of recording elements are arranged in one recording head to reduce the recording time as short as possible is mainly used as a recording head for use in a printer, a facsimile apparatus, a copying apparatus, or the like. Generally, however, in the case where the recording elements of one multi recording head have been driven under the same driving condition from a viewpoint of a manufacturing problem, an uneven image density occurs in a recording image due to a variation in recording characteristics by the recording elements, so that a recording image of good quality is not achieved. Therefore, an apparatus to prevent the above uneven image density has been proposed. The above apparatus has: memory means for printing a test pattern of a certain predetermined density for a recording head in the case where recording characteristics of recording elements are uneven and for calculating and storing data according to recording characteristics of the recording elements; and means for correcting input image data on the basis of the memory means and the stored data.
For instance, a multinozzle head of the ink jet type has a multihead 61 which is constructed by arranging recording elements 62 in a line as shown in FIG. 6A. In the case where input signals to the image recording elements are set to be uniform as shown in FIG. 6B, an uneven image density as shown in FIG. 6C occurs as an example. In this case, the input signals are corrected as shown in FIG. 6D, a large input signal is given to the image recording elements in the portion of low densities, and a small input signal is given to the image recording elements in the portion of high densities. In the case of a recording system which can modulate a dot diameter or a dot density, a diameter of dot which is recorded by each image recording element is modulated in accordance with the input signal. For instance, in the ink jet recording system of the piezoelectric type, a driving voltage or a pulse width which is applied to each piezoelectric element, while in the case of the thermal copy transfer recording system, a driving voltage or a pulse width which is applied to each heating element is changed in accordance with the input signal, thereby making uniform the dot diameters or dot densities by the recording elements. Thus a density distribution is made uniform as shown in FIG. 6E. On the other hand, in the case where it is impossible or difficult to modulate the dot diameter or dot density, the number of dots is modulated in accordance with the input signal. A larger number of dots are printed by the image recording elements in the low density portion and a small number of dots are printed by the image recording elements in the high density portion, so that a density distribution is made uniform as shown in FIG. 6E.
A correction amount in the above cases is obtained, for instance, by the following method.
The case of correcting an uneven image density of a multihead comprising 256 nozzles will now be described as an example. FIG. 7 shows an uneven image density distribution which occurs when dots have been printed by a certain uniform image signal S. First, a mean density OD of the head is obtained. Then, densities OD.sub.1 to OD.sub.256 of the portions corresponding to the nozzles are measured. Subsequently, .DELTA.OD.sub.n =OD-OD.sub.n (n=1 to 256) are obtained. Now, assuming that the relation between the value of the image signal and the output density, that is, the tone characteristic is linear as shown in FIG. 8, it is proper to correct the image signal by only .DELTA.S in order to correct the density by only .DELTA.OD.sub.n. For this purpose, it is sufficient to execute a table conversion as shown in FIG. 9 to the image signal. In FIG. 9, reference numeral 301 denotes a straight line of an inclination of 1.0. An input signal is generated without being converted. On the other hand, reference numeral 302 denotes a straight line of an inclination of (S-.DELTA.S)/S and when an input signal of S is given, a signal of S-.DELTA.S is generated.
Therefore, if the head is driven after executing a table conversion as shown by the straight line 302 in FIG. 9 for the image signal corresponding to the n-th nozzle, a density of the portion which is printed by the n-th nozzle is equal to OD. By performing such a process for all of the nozzles, the uneven image density is corrected and a uniform image is derived. That is, the uneven image density can be corrected by previously obtaining data indicating which table conversion should be optimally performed to the image signal corresponding to which nozzle.
However, in the above conventional example, actually, the tone characteristics of all of the nozzles are not expressed by the straight lines as shown in FIG. 8. Drawbacks in the case where the tone characteristics of the nozzles differ will now be explained. The case of correcting a difference of the image densities between two nozzles will now be considered for simplicity of explanation.
In FIG. 10, reference numeral 401 denotes a tone characteristic of a nozzle-1 and 402 indicates a tone characteristic of a nozzle-2. Since an ink discharge amount of the nozzle-2 is larger than that of the nozzle-1, the nozzle-2 has such a tone characteristic. In the case of correcting the density difference for the input signal S as mentioned above, the image signal for the nozzle-2 is increased by (S-.DELTA.S)/S times. Thus, the tone characteristic 402 is extended in the X direction by only S/(S-.DELTA.S) times. The extended tone characteristic 402 is as shown at 402' in FIG. 11.
Although the density difference for the input signal S has been corrected, density differences still remain in the other regions. To correct the density differences for all of the regions, it is necessary that all of the tone characteristics of the nozzles are expressed by straight lines.
However, since the ink emission amounts of the nozzles actually differ, the tone characteristics of the nozzles also differ.
Therefore, there is a drawback such that even if an uneven image density could be corrected at a certain print duty, uneven image densities remain at the other duties and it is very difficult to correct the uneven image densities for the whole region.
The above drawback is not limited to the foregoing ink jet system. Even in the other systems such as thermal copy transfer system, LED printer, and the like, there is a variation of dot diameters which are printed by the heating elements and there is also a variation of tone characteristics. It is, therefore, also similarly very difficult to correct the uneven image densities.